Liver function can be assessed by administering an exogenous substance to quantify changes in hepatic blood flow, uptake, biotransformation, and excretion. Characterization of drug half-life, clearance, and product formation rates are possible methods for measuring hepatic efficiency. Allopurinol and caffeine have been used to measure metabolite formation followed by renal elimination of both parent substance and metabolite. Sorbitol, a substance with high intrinsic clearance, can reflect liver blood flow, while trimethadione, a low-extraction drug, has been used to measure liver enzyme capacity. Metabolites from lidocaine, methacetin, and aminopyrine have been measured in serum, urine, and breath tests. Salivary clearance measurements of caffeine and antipyrine are reported as suitable for routine use. Genetic diversity of isoenzymes and the many metabolic processes used by hepatocytes make it extremely difficult to quantify functional changes with one substance. Combinations of model substrates have been suggested to assess the many hepatic processes.